The present invention relates to a micromechanical diaphragm having a partially n-doped p-substrate on its surface, a topmost layer being an n-epitaxial layer.
In micromechanical pressure sensors, a diaphragm is defined by anisotropic etching with a pn-etch stop. The etch front stops at the boundary surface between the p-doped substrate and an n-doped layer. The clamping of the diaphragm is defined by the edge of the etched pit. The pressure-dependent deflection of the diaphragm is measured via the change in resistance of piezoresistive resistors on the surface of the diaphragm. Of critical importance for the sensitivity of the resistors is the position of the resistors relative to the clamping of the diaphragm.
A micromechanical diaphragm is already known from German Published Patent Application No. 43 09 207. This publication describes a semiconductor device with a piezoresistive pressure sensor having a diaphragm formed by a conductive epitaxial layer (epitaxy layer) and applied to a semiconductor substrate of opposite conductivity. At least one piezoresistor is provided on the diaphragm surface facing away from the semiconductor and an opening penetrating the semiconductor substrate has been etched into the inner surface of the diaphragm. A conductive intermediate layer having an annular structure has been inserted between the semiconductor substrate and the epitaxial layer, the intermediate layer defining the area of the opening adjacent to the inner surface of the diaphragm. The intermediate layer possesses a conductivity which is opposite to that of the semiconductor substrate. This diaphragm with varying dopings ensures that there are only limited differences in thickness in the diaphragm which are determined by the penetration depth of the doping. In addition, the diaphragm described here has only one epitaxial layer.
An object of the present invention is to design and arrange a micromechanical diaphragm in such a way that precisely formed clampings or diaphragm areas with strongly varying stiffness are ensured.
This object is achieved according to the present invention in that one or more n-epitaxial layers which are p-doped in the diaphragm area are arranged on the p-substrate. This ensures that the clamping or support points of the diaphragm are defined during the subsequent etching of the diaphragm, i.e. the p-doped area, and not by the underetching of the lateral surfaces. The edge area of the etching within the p-doped substrate is located below the additionally arranged n-epitaxial layers and does not influence the behavior or the clamping of the diaphragm. A precise definition of the clamping of the diaphragm is, for example, very important for pressure sensors since the position of the piezoresistive resistors relative to the clamping (diaphragm edge) influences the sensitivity with respect to pressure.
It is advantageous in this regard that the substrate is locally n-doped before the n-epitaxial layer is deposited and that the n-epitaxial layer is n-doped during the deposition and is then locally p-doped or p-doped during the deposition and then locally n-doped in the edge area or the epitaxial layer is deposited undoped and subsequently the edge area is provided locally with an n-doping and the center area of the epitaxial layer is provided with a p-doping. This type of doping makes continuous doping possible so that the individual n-epitaxial layers are thicker. The variable number of n-epitaxial layers presented here also permits clearly greater thicknesses of the clamping of the diaphragm.
According to a further development, an additional possibility is that an n-doped diaphragm layer or the diaphragm is in contact with the locally p-doped n-epitaxial layer or the locally n-doped p-substrate. The diaphragm thus formed or stiffening of the diaphragm is substantially thicker in the clamping area and accordingly significantly more stabile.
In addition, it is advantageous that the p-doped area of the various n-epitaxial layers or of the locally n-doped p-substrate is formed with a varying size in the individual n-epitaxial layers or in the doping layer or extending from the diaphragm layer or a diaphragm, it has a larger or smaller surface area than in the preceding layer. It is thus possible to design any desired stiffening form below the diaphragm and the diaphragm can be adapted corresponding to its field of application.
It is also advantageous in this regard that the p-doped area of the various n-epitaxial layers or of the locally n-doped p-substrate is arranged in various subareas of the respective layer, such as the center of the micromechanical diaphragm or below the diaphragm and the n-doped areas and the p-doped areas, always in alternation with an n-epitaxial layer, are arranged side by side and/or symmetric to the center of the micromechanical diaphragm.
According to a preferred embodiment of the device according to the present invention, it is also provided that the various n-epitaxial layers are formed with varying thicknesses.
It is of particular significance for the present invention that the areas previously identified as n-doped may also be p-doped and the areas previously identified as p-doped may also be n-doped. Accordingly, the base doping of the various layers may be selected freely and the corresponding locally provided doping may be adapted.
Finally, it is of advantage that the diaphragm layer is formed as an n-epitaxial layer and the side of the p-substrate directed to the outside has an etch mask which opens up the area to be exposed below the diaphragm to the etching agent.
Lastly, it is of advantage that one side of a p-substrate is locally n-doped in the edge area; an n-epitaxial layer is deposited on or applied to this layer, the center of the n-epitaxial layer being p-doped corresponding to the preceding substrate layer, and a second n-epitaxial layer is applied corresponding to the first n-epitaxial layer, the second n-epitaxial layer being p-doped in the diaphragm area or in additional areas. After that, a purely n-doped diaphragm layer is deposited on or applied to the n-epitaxial layer, an etch mask being applied to the exposed surface of the substrate.